1. Field of the Invention
The present invention relates to a novel system for the purification of nitrous oxide. The present invention also relates to a method of producing nitrous oxide of ultra-high purity.
2. Description of the Related Art
In the semiconductor manufacturing industry, nitrous oxide (N2O) is employed as a chemical vapor deposition (CVD) process gas for depositing films such as silicon oxide and silicon oxynitride. The use of a highly pure material is important to prevent contamination of the wafers and other deleterious effect on the processes being run ultimately resulting in degraded yield of the final product.
It is particularly important that the nitrous oxide be of ultra high purity, on the order of 99.999% or greater purity. Such a purity will ensure uniform film growth during deposition.
A conventional nitrous oxide purification system and process is described in European Patent Document No. 0 636 576 A1. This document discloses compressing a nitrous oxide feed gas that contains nitrogen, oxygen and water in a compressor. A portion of nitrous oxide, free of water, is conveyed to a heat exchanger where it is condensed to a liquid. The liquid gas is directed to the top of a rectification column for further condensation and returned to the bottom of the rectification column. A second mixed gas-liquid nitrous oxide portion is directed from the heat exchanger to the bottom of the rectifying column as a heating source to vaporize the liquid. The same nitrous oxide is then directed out of the bottom of the column and introduced to the middle of the column. The final product is taken from the bottom of the column having the requisite purity.
One of the disadvantages associated with such a system is that it is far too complex, as it includes three heat exchangers and two rectification sections. In addition, the compressor employed is potentially a secondary source of contamination due to the lubricating oils needed to operate it.
A further disadvantage associated with the system described above and other related art systems is that they are not capable of removing carbon dioxide impurity. Carbon dioxide is known to those skilled in the purification art to be one the most difficult impurities to remove from nitrous oxide. Additionally, although useful in removing nitrogen, oxygen and water impurities, the systems of the related art are not capable of removing other impurities such as carbon monoxide, ammonia, methane and hydrogen. These impurities are hereinafter referred to as light impurities.
To meet the requirements of the semiconductor manufacturing industry and to overcome the disadvantages of the related art, it is an object of this invention to provide a novel system and method for purifying nitrous oxide gas. The invention allows for the removal of impurities and particulates, such that an ultra pure nitrous oxide product having a purity of 99.9998% or higher can be delivered to a point of use.
A further advantage associated with the novel system is that the total cost of purification can be significantly reduced by eliminating complicated secondary operations needed to eliminate carbon dioxide and light impurities other than nitrogen, oxygen and water. In addition, cost and time associated with employment of a distillation column having a number of rectification sections and heat exchangers is eliminated.
It is a further object of the invention to provide a cryogenic distillation design system that reduces the impurities trapped.
It is yet another object of the invention to obtain total reflux in the distillation system due to the design of the condenser and distillation column.
It is a further object of the invention to provide a batch process for the removal of light impurities from nitrous oxide.
It is another object of the invention to provide a continuous process for the removal of acid gases from nitrous oxide.
It is a further object of the invention to minimize the amount of nitrous oxide lost during the process.
It is another object of the invention to provide a purification system wherein the components can be easily accessed to be repaired or replaced in a quick and facile manner.
It is a further object of the invention to provide a system of delivery for ultra pure nitrous oxide.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art on a review of the specification, drawing and claims appended thereto.
In accordance with the present invention, innovative methods and systems for purification of nitrous oxide is provided. The invention finds particular applicability in the semiconductor manufacturing industry, wherein ultra pure chemical gases generated are introduced directly into one or more semiconductor processing tools.
According to a first aspect of the invention a method for removal of light impurities from a nitrous oxide, for a semiconductor production application is provided. The method includes the steps of:
(a) providing a liquefied nitrous oxide in a purification tank;
(b) routing the liquefied nitrous oxide gas from the bottom of the purification tank to a vaporizer, wherein the liquefied nitrous oxide is vaporized to form nitrous oxide vapor;
(c) communicating the nitrous oxide vapor to a first end of the purification tank and;
(d) conveying the nitrous oxide vapor from the first end and allowing it through the entire vapor space to a second end of the purification tank into a distillation column and a condenser, wherein the light impurities are removed.
In accordance with a further aspect of the invention, a method for nitrous oxide purification, wherein the nitrous oxide product is employed for semiconductor manufacturing. The process includes the steps of:
(a) converting a liquefied nitrous oxide devoid of light impurities to a gas by passing the liquefied nitrous oxide through a vaporizer;
(b) routing the nitrous oxide gas to a first dry bed vessel, wherein an acid gas is absorbed; and
(c) conveying the nitrous oxide gas from the first dry bed vessel to a second dry bed vessel, wherein water and ammonia are removed.
In accordance with a further aspect of the invention a method for delivery of a purified nitrous oxide gas in a liquefied state is provided. The method includes:
conveying a purified nitrous oxide from a product tank to at least one holding vessel via a manifold system.
In accordance with yet another aspect of the invention a system for removing light impurities from a nitrous oxide gas is provided. The system includes: (a) a purification tank for holding a liquefied nitrous oxide, having a first end a second end; (b) a vaporizer in communication with the first end of the purification tank to receive the liquid, vaporize and convey a nitrous oxide vapor back to the first end of the purification tank; (c) a distillation column disposed.on the second end of the purification tank to receive the nitrous oxide vapor; and (d) a condenser disposed on the distillation column, wherein light impurities are removed out of the vent on top of the column.
In accordance with another aspect of the invention a system for nitrous oxide purification is provided. The system includes: (a) a vaporizer for receiving and converting a liquefied nitrous oxide devoid of light impurities to a vapor; (b) a first dry bed vessel disposed downstream of the vaporizer to receive the nitrous oxide vapor and absorbing an acid gas therein; and (c) a second dry bed vessel downstream of the first dry bed vessel for removing water and ammonia therein.
In accordance with yet another aspect of the invention a system for delivery of a purified nitrous oxide gas. The system includes a product tank for holding the purified nitrous oxide gas and a manifold for distributing the purified nitrous oxide to at least one holding vessel.
Thus, in accordance with the inventive system light impurities are removed within a first sub-system where the process is carried out in a batch type manner. The acid gases and moisture are removed within a second subsystem in a continuous manner and the ultra pure nitrous oxide is delivered via a third-system to the point of use.